Accelerator mass spectrometry (AMS) 14C dating off rock varnish provides minimum-limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods. Experiments on sample collection, the effect of different individuals preparing samples, the influence of different chemical-extraction procedures, the incorporation of carbonate detritus, possible contamination from rock underlying varnish, the role of bio-geochemical erosion of varnish, and other influences reveal procedures that yield reproducible results. Conventional radiocarbon dates from charcoal beneath lava flows of Hualalai Volcano, Hawaii, and from arid sites in western North America provide controls to test varnish radiocarbon dating. AMS radio-carbon ages on the very bottom layer of varnishes are typically ≤10% younger than the 14C ages of these controls. Applications are illustrated here for fluvial, aeolian, peri-glacial, hillslope, lacustrine, and glacial geo-morphology, as well as rock-art research in archaeology.